This application is a 371 of PCT/EP02/04040, filed Apr. 11, 2002, which claims the priority of German Patent Application No. 101 28 414.4, filed Jun. 12, 2001, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to an exhaust gas purification unit for purifying the exhaust gas from an internal combustion engine which is preferably operated in lean burn mode.
A known exhaust gas purification unit is described in DE 40 32 085 A1. The corresponding exhaust gas purification installation includes an arrangement of denox or SCR (SCR=selective catalytic reduction) catalytic converters. These SCR catalytic converters catalyze a selective reduction reaction, in which nitrogen oxides are reduced under oxidizing conditions to form harmless nitrogen with the aid of ammonia which is introduced into the exhaust pipe or a reducing agent which releases ammonia. Ammonia is used as reducing agent, and additionally the upstream connection of a particulate filter for removing condensed exhaust gas constituents is proposed. These and similar installations remove the nitrogen oxide gaseous pollutants from oxygen containing exhaust gases, with the catalytic activity typically being effective in a temperature range between 200° C. and 450° C.
As statutory exhaust gas limit values are reduced further, effective lowering of the levels of other harmful exhaust gas constituents, such as for example carbon monoxide, hydrocarbons, has to be achieved in addition to the reduction in the levels of nitrogen oxides, in particular at relatively low exhaust gas temperatures.